This invention relates to an improved self-contained downhole blowout preventer which is placed in a drill string and operates, upon control from the surface, to isolate a geologic formation by sealing off the annular space between a drill string and a borehole wall. It particularly relates to an acoustically operated electro-mechanical downhole blowout preventer which may be in the shape of a drill collar.
The equipment used in the rotary drilling of oil and gas wells consists of a long drill string which may extend many thousands of feet into the earth, a rotary table to turn the drill string, a derrick and its associated hoisting equipment, and the drill bit. The drill string itself is merely a series of detachable hollow pipe sections terminated at its lower end by the drill bit. This lengthy drill string requires two seemingly opposite forces acting upon it for proper operation. The drill string is kept in tension to prevent buckling or kinking by suspending it from rig hoisting equipment in the derrick. Conversely, the drill bit must have substantial downward pressure placed on it or the desired drilling does not take place. For this reason drill collars make up the lower end of the drill string just above the drill bit. Drill collars are thick-walled, heavy sections of pipe which replace some pre-determined length of drill pipe and result in a stable drill string with a properly weighted drill bit.
In using the described equipment to produce a borehole, it should be apparent that significant amounts of heat and drilling cuttings are produced. Both are removed by a drilling fluid often called "mud". The mud enters the hollow drill string at the surface, proceeds down through the drill pipe and drill collar sections, and finally exits the drill string through specially provided nozzles in the drill bit. As the mud leaves the drill string, it cools the drill bit and picks up whatever cuttings may be present. Some drill bit designs also rely upon the potentially significant hydraulic force of the mud exiting the bit nozzles to add to the overall drilling effect. In either case, the mud proceeds up the annular space found between the drill string and the borehole wall to the surface. The mud is then screened, reconstituted, and recycled to the drill string.
Drilling mud can serve a number of other important functions as it cycles through the well. For instance, it may be used to control relatively low or relatively high subsurface pressures. Drilling a well to any significant depth often requires the bit to penetrate rock and sand strata of widely varying composition. Certain depleted Gulf Coast gas sands have localized or formation pressures that are so low and porosities that are so high that a sizeable portion of the drilling mud disappears into these strata rather than returning to the surface. The addition of certain clays, such as bentonite, to the drilling mud will often remedy the situation. Bentonite is a clay which, when included in the drilling mud using proper techniques, forms a dispersed mixture of flat platey particles. These particles form a thin and relatively impermeable filter cake on the borehole wall in the region of the offending "thief" stratum. The filter cake substantially prevents further loss of mud into the stratum and allows circulation of the mud to resume.
At the other end of the pressure control spectrum lies the problem of containing a high pressure fluid within the well. For instance, when a subsurface formation containing a high pressure fluid is penetrated, the localized pressure of the fluid may well exceed the liquid or hydraulic head of the mud lying above it. When this occurs, a "kick" is observed and may eventually result in a "blowout" of the drilling mud from the wellbore annulus. A mud having a higher specific gravity, and resulting in a correspondingly higher hydraulic head at the downhole drilling site, is introduced into the well. The higher density mud is intended to keep the formation fluid in its stratum.
Since the well is sealed off at the surface, the possibility exists that a significant volume of high pressure formation fluid will enter and fill the entire wellbore. If such occurs, downhole pressure may increase to the point that the integrity and safety of the well is threatened. If the formation fluid influx can be isolated lower downhole, then the pressure build-up could be minimized, the formation fluid influx minimized, and the integrity of the well maintained. A downhole BOP suitable for isolating a high pressure stratum, however, is difficult to design in that it must fit inside a relatively narrow borehole, be compatible with the drill string and its auxiliary equipment, and remain reliably operative during infrequent emergencies.
A downhole BOP can be mounted just below a circulation sub which allows fluid communication between the pipe bore and the annulus. The BOP would, therefore, seal off the annulus with an exterior bladder or packer at a point in the borehole above the flowing formation. A heavy weight or "kill" fluid could then be circulated down the drill and back up the annulus above the BOP utilizing the circulation sub. Once the heavy weight drilling fluid is in position and able, by its hydraulic head, to maintain the formation fluid in its stratum, the downhole BOP could be deflated and drilling re-commenced.
The disclosed downhole BOP is especially suitable for physically isolating a stratum containing high pressure fluid during the interim between observation of a kick and circulation of the high density mud to the wellbore annulus.
The operation of a known downhole packer is found in U.S. Pat. No. 2,779,419, to Mounce.
A circulation sub particularly suitable for use with the BOP disclosed herein is found in Ser. No. 218,602, filed Dec. 19, 1980, to Bednar et al, the entirety of which is incorporated by reference.
The few known circulation subs and downhole BOPs typically have been mechanically actuated. That is to say that the driller sets these devices in motion other than through acoustic, electrical, or hydraulic means. A representative of such a mechanically actuated combination is found in U.S. Pat. No. 3,941,190, to Conover. The circulation sub, disclosed in conjunction with a downhole packer suitable for use as a blowout preventer, utilizes a sleeve valve to cover the required drilling fluid circulation ports. The sleeve valve is kept in a position covering the ports during normal drilling operations by a spring. Actuation of the circulation sub requires the insertion of a metal plug known as a "go-devil" into the pipe bore from the surface. The go-devil falls to a seat in the vicinity of the sleeve valve and utilizes the localized pressure in the pipe bore to slide the seated go-devil and sleeve valve down and away from the mouth of the circulation ports. In order to re-start drilling operations, the driller must fish the go-devil out of the drill string along with some associated packer machinery which plugs the pipe bore.
Other representative inflatable packers, which are actuated by devices inserted into the drillpipe and dropped to the vicinity of the packers, are found in U.S. Pat. No. 3,529,665 and U.S. Pat. No. 3,606,924, both to Malone, and U.S. Pat. No. 3,850,240, to Conover.
As noted above, the inventive BOP is desirably acoustically actuated. Acoustically operated downhole devices are known. For instance, U.S. Pat. Nos. 3,961,308, 4,073,341, and 4,129,184, all to Parker, discuss the use of acoustic waves to actuate the disaster valves often found in the tubing of product wells to shut off the flow of hydrocarbon fluids. However, the application of acoustic actuation to drill pipe and drill collars in a non-cased well is not a trivial matter. The disclosed invention in its acoustically operated configuration is capable of operation in wells which are cased or non-cased.
The theoretical use of torsionally propagated acoustic waves in the drill string to provide downhole communication is suggested in a paper entitled "A New Approach to Drill-String Acoustic Telemetry" by Squire and Whitehouse. This paper, SPE 8340, was presented at the Fall 1979 conference of the Society of Petroleum Engineers of AIME.
It is apparent in this prior art that there is no disclosure or suggestion concerning the unique valving configuration utilized in controlling the inventive BOP. Similarly the use of electric motors, acoustically actuated or not, to control such a valving configuration is not shown.